The Vishnu Sahasranaamam was first chanted by Shri Bhishma as he lay on his bed of arrows in the battlefield of Kurukshetra. Its authenticity is further bolstered by the fact that Lord Krishna Himself was present there during the initiation of Yudhishthira with the Vishnu Sahasranaamam.
The Vishnu Sahasranaamam or the "Thousand Names of Vishnu" is one of the most popular Stotra's in Sanathana Dharma. "Naama can crush a mountain of sins, Naama can break the neck of death itself," says the Sai Satcharita. Shri Bodhendra Saraswathi Swamigal, the 59th Shankaracharya of the Kanchi Kamakoti Peetham has spoken extremely highly of the good achieved by Naama Sankeerthanam (or "singing of the Name").
The Vishnu Sahasranaamam may be chanted by all, without bar. There is even a Shloka in the Phala Shruti (or "Benefits of Chanting Section") of the Vishnu Sahasranaamam which may be chanted thrice if there is, on rare occasion, no time to chant the Sahasranaamam in its entirety. This Shloka goes:
"Sri Raama Raama Ramethi Ramey Raamey Manoramey. Sahasranaama Thath Thullyam Raama Naama Varaananey." - "Chanting Rama's name thrice is equivalent to chanting the Vishnu Sahasranaamam."
Bhima once described Lord Krishna's name to be greater than Krishna Himself for he can always be in the company of the Lord in his Mantra Svaroopa (Mantric Form) even if not in his Moola Svaroopa (True Form).
Provided here (Sanskrit, English Transliteration & English meanings) is a PDF document containing the Vishnu Sahasranaamam.